Process for measuring the inclination of boundary areas in an optical system using interferometry to extract reflections from disturbance-generating boundary areas

ABSTRACT

The description relates to the measurement of the inclination of boundary areas in an optical system and a device for implementing the process. Starting with an auto-collimation process which determines the inclination of this boundary area in relation to a reference axis from the deviation of a light beam collimated onto the boundary area to be investigated and reflected on a position-sensitive photodetection system, a twin-beam interferometer process with downstream evaluation electronics is provided which permits the separation of the interferogram, intensity-modulated according to the invention, of the boundary area to be investigated from the unmodulated disturbance reflections of those not to be investigated as far as the interferogram intensities in the photon-noise range and thus offers a resolution in the deviation of the center of gravity of the interferogram of the order of 10 nm. To modulate the intensity of the interferogram, the difference in the optical distance between the reference and test beams to the boundary area to be investigated is time-modulated.

BACKGROUND OF THE INVENTION

The invention relates to a process and a device for the measurement ofthe inclination of boundary areas in an optical system with respect to areference axis.

A device of this type is disclosed in EP-A2 0,253,242. In the devicedescribed in that document, a light beam which is emitted from a lightsource and which is bounded by a diaphragm is collimated by an opticalimaging system onto a boundary area which is to be investigated and theinclination of which in relation to a reference axis is to bedetermined. The wavefront, reflected by the boundary area on which it iscollimated, is imaged by the same optical imaging system onto aposition-sensitive photodetection system. From its position-dependentoutput signals, an electronic evaluation system determines the deviationof the diaphragm image reflected by the boundary area to be investigatedin relation to the reference axis, and thus the inclination of theboundary area to be investigated in relation to the reference axis. Inthe described device, a CCD line sensor is employed as aposition-sensitive photodetection system. A special threshold valuecircuit is provided, in order to separate the measurement signalsassociated with the diaphragm image from the heterodyned disturbanceradiation signals.

In place of the line sensor, it is also possible to use a four-quadrantdiode or a lateral diode, as is realized in the electronicauto-collimator UDT model 1000 from the company United DetectorTechnology.

A disadvantage in the case of the measurement of the inclination ofboundary areas using auto-collimators occurs in the case oflow-reflection boundary areas, since in this case the position of thediaphragm image can no longer be reliably measured, as a result of thepoor contrast. Likewise, the method proves to be critical where radiicenters of various boundary areas lie closely one behind the other. Inthese circumstances, the boundary areas with the adjacent radii centersdisturb the measurement as a result of extraneous light. There is eventhe danger of the confusion of the boundary area to be investigated witha boundary area which is not to be investigated, i.e. an unambiguouscorrelation of the diaphragm reflection with a specified boundary areais not possible.

SUMMARY OF INVENTION

The object of the invention is to provide a process and a device formeasuring the inclination of boundary areas in an optical system, whichprocess and device permit an unambiguous separation of the reflection ofthe boundary area to be investigated from the disturbing reflections ofthe boundary areas which are not to be investigated, permit a detectionof the reflection intensity down to the limit of the photon noise, andoffer a resolution of the deviation of the reflection from a referenceaxis in the order of magnitude of 10 nm.

In the case of a process of the initially mentioned type, this object isachieved according to the invention by providing a method of measuringinclinations of boundary areas in an optical system, in which a lightbeam is collimated onto a boundary area which is to be investigated ineach instance and, from a deviation of a wavefront reflected by theboundary area to be investigated in relation to a reference axis on aposition-sensitive photodetection system, the inclination of theboundary area to be investigated in each instance in relation to thereference axis is determined. The method includes the steps of: a)splitting the light beam into a test beam and a reference beam; b)guiding the reference beam via a reference beam path onto a referencemirror; c) adjusting an optical path length of the reference beam to asame optical path length as the test beam as far as the boundary area tobe investigated in each instance; d) selecting the coherence length ofthe light beam to be shorter than a shortest optical path length betweenthe boundary area to be investigated in each instance and the closestdisturbance-reflection-generating boundary area of the optical system;e) time-modulating the optical path length difference between thereference beam and the test beam as far as the boundary area to beinvestigated with a range of modulation not greater in magnitude thanthe coherence length; f) imaging a wavefront reflected by the boundaryarea to be investigated in each instance onto the position-sensitivephotodetection system where it is heterodyned with a wavefront reflectedby the reference mirror, thereby producing on the position-sensitivephotodetection system a temporally intensity-modulated interferogram;and g) electronically evaluating position-dependent signals of theposition-sensitive photodetection system to separate a modulated signalcomponent generated by the temporally intensity-modulated interferogramfrom unmodulated signal components generated by disturbance reflectionsand the reference beam and thereafter determining a deviation of thecenter of gravity of the interferogram in relation to the referenceaxis. device for implementing this process includes: a light source; aposition-sensitive photodetection system; an optical imaging systemwhich collimates a light beam, emitted by the light source, onto theboundary area of the optical system and images a wavefront, reflectedfrom the bound area onto the position-sensitive photodetection system;an electronic evaluation system connected to the position-sensitivephotodetection system, the electronic evaluation system determining adeviation of a wavefront reflected by the boundary area in relation to areference axis and thus of the inclination of the boundary area inrelation to the reference axis; an auxiliary optical system arranged togenerate a parallel light beam and to image the wavefront reflected bythe boundary area onto the position-sensitive photodetection system; abeam splitter for splitting the parallel light beam into a test beam anda reference beam; an optical collimation system arranged to collimatethe test beam onto the boundary area; a reference mirror arrangedperpendicular to an optical axis of the reference beam; an adjustmentdevice for balancing the optical path length of the reference beam tothe optical path length of the test beam as far as the boundary area;the light source having a coherence length shorter than the shortestoptical path length between the boundary area and a closestdisturbance-reflection-generating boundary area of the optical system; amodulator, the modulator time-modulating the optical path length of thetest beam or the reference beam with a range of modulation no greater inmagnitude than the coherence length such that an intensity-modulatedinterferogram of interference between the test beam and the referencebeam is produced; a two prism system arranged to return a wavefront,reflected by the reference mirror, to the position-sensitivephotodetection system, wherein the electronic evaluation systemprocesses position-dependent signals output by the position-sensitivephotodetection system to separate a modulated signal component generatedby the intensity-modulated interferogram from unmodulated signalcomponents generated by the disturbance reflections and unmodulatedsignals of the reference beam, and thereafter determines the deviationof a center of gravity of the interferogram in relation to the referenceaxis. Advantageous further developments of the process as well asadvantageous refinements of the device are evident from the followingdescription and appended claims.

DESCRIPTION OF THE DRAWINGS

The process according to the invention and the device according to theinvention are described hereinbelow with reference to an embodimentshown diagrammatically in the drawings, for a suitable interometerarrangement in which:

FIG. 1A-1C show embodiments of the device according to the instantinvention with like reference numbers indicating like parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The application of interferometric methods for measuring theinclinations of boundary areas in optical systems is successfullyintroduced by the invention, since in the case of the measurement of theinterferogram resulting from the boundary area collimated onto, which isto be investigated, the disturbing reflections caused by the boundaryareas which are not of interest and which are not collimated onto areseparated. As a result of this, a reliable correlation of theinterferogram with the associated boundary area becomes possible.

On account of the time modulation in one of the beam paths, the processis to be designated as a heterodyne process. In the case of theheterodyne interferometric processes, use is made of the fact that thesignal radiation and the disturbance radiation arise at differentlocations. If now the signal radiation and the disturbance radiation aredifferently labeled, it is also possible to distinguish between them inthe course of the signal evaluation.

To this end, in the process according to the invention the light beam isdirected parallel and thereafter split up at a beam splitter into a testbeam and a reference beam. The test beam is then collimated onto theboundary area which is to be investigated in each instance. On accountof the optical data of the optical collimation system employed for thispurpose and of the data of the optical system to be investigated, thetest beam may be collimated onto a specified boundary area. Collimationonto a plane boundary area is understood as indicating that the testbeam is imaged at infinity, while in the case of collimation onto aspherical area the imaging takes place onto the center of radii of thatarea. The wavefront of the test beam, which wavefront is reflected bythe boundary area to be investigated, is to be designated as the signalbeam. As against this, those components of the test beam which arereflected by the other optical boundary areas which are not to beinvestigated are to be designated as disturbance beams or disturbancereflections. The wavelength of the radiation employed is to be adaptedto the optical properties of the boundary areas to be investigated andis not restricted to light in the conventional sense. Rather, by way ofexample, it is also possible to use radar, mm and ultrasonic waves.

The reference beam is thrown back into itself via a reference mirrorwhich is preferably disposed perpendicular to its optical axis and ispreferably conducted as a parallel beam onto the position-sensitivephotodetection system.

The balancing of the optical path length of the reference beam with thatof the test beam as far as the boundary area of the optical system whichis to be investigated in each instance takes place by displacement ofthe reference mirror.

After this, the wavefront reflected by the boundary area of the opticalsystem which is to be investigated is imaged by an auxiliary opticalsystem onto the position-sensitive photodetection system and is thereheterodyned with the wavefront reflected by the reference mirror, sothat an interferogram is constructed. The center of gravity of theinterferogram lies in a deviation in relation to the reference axis,which deviation corresponds to the angle of inclination.

As a result of the selection of a coherence length of the light beam,which coherence length is shorter than the smallest optical pathdifference between the signal beam and the disturbance beams, there isno interference of disturbance beams with the reference beam. However,the disturbance reflections have not yet been eliminated in this way.They continue to be heterodyned with the interferogram.

In order to be able to distinguish the interferogram of the boundaryarea of the optical system which is to be investigated in each instancefrom the disturbing reflections of the other boundary areas which arenot to be investigated and which are disturbing boundary areas, theinterferogram is intensity-modulated. On this basis, for the processingof the signals of the position-sensitive photodetection system it islabeled in a distinguishable manner, since the disturbance beams andthus also the disturbance reflections are not intensity-modulated.

According to the invention, the intensity modulation of theaforementioned interferogram is generated by time modulation of theoptical path length of the test beam or reference beam. As a result ofthis, the optical path length difference between the reference beam andthe test beam is modulated up to the boundary area to be investigated.In order, in this case also, to exclude an interference of disturbancebeams and reference beam, the range of modulation must not exceed themagnitude of the coherence length which is selected according to theinvention.

A modulation of the reference beam path has been known per se for agreat length of time. Thus, the patent specification DE 2,528,209 B2describes, for the measurement of optical surface areas having areflectance of at least a few per cent, an optical sensitive tracer inwhich, in a twin-beam interferometer, the path difference between thetest beam path and the reference beam path is modulated by more than thecoherence length of the light. However, in the described opticalsensitive tracer the modulation of the reference beam path is notundertaken in order to label a specified light beam or an interferogramof specified origin. Rather, the modulation in that case serves for thegeneration of an internal scale of extension for the measurementprocess. In this process, the measurement principle employed is theconsideration that at the periodically recurring instance of maximumcontrast of the white light interferences, to which a maximum modulationdepth of the receiver signal corresponds, the instantaneous range ofmodulation indicates the distance of the measurement object.Accordingly, the range of modulation must be measured in the opticalsensitive tracer.

In contrast to this, in the present invention a measurement of the rangeof modulation is not required, and the range of modulation must fallbelow the coherence length. The time modulation of the optical pathlength of the test beam or reference beam only within the--as alreadydescribed above--selected short coherence length offers the advantagethat only a single intensity-modulated interferogram of the signal beamand reference beam is created, which brings about a time-modulatedsignal at the position-sensitive photodetection system. With this thereis heterodyned a time-unmodulated photodetector signal which resultsfrom the disturbance beams and the reference beam, since the disturbancebeams cannot interfere with the reference beam, on account of theselected short coherence length. In these circumstances, thetime-modulated photodetector signal component can easily be separatedfrom the time-unmodulated photodetector signal component by electronicevaluations.

In the first instance, the electronic evaluation system separates thesignal components of the temporally intensity-modulated interferogramfrom those of the intensity-unmodulated disturbance reflections and ofthe intensity-unmodulated reference beam. Advantageously, to this enduse is made of the lock-in technique with utilization of the timemodulation of the interferogram, with which technique it is alsopossible to separate an interferogram with an intensity far below thatof the disturbance reflections, so that, for example, when investigatingextremely low-reflection boundary areas a detection of the signal beamreflection intensity down to the limit of the photon noise is possible.Irrespective of the selected range of modulation, the lock-in processingis undertaken on a phase-independent basis with a working frequencywhich is equal to the modulation frequency.

In the second step, the electronic evaluation system determines thedeviation of the center of gravity of the interferogram in relation tothe reference axis and thus the inclination of the boundary area to beinvestigated.

The time modulation of the optical path length of the test beam orreference beam can take place by a pure path length modulation, e.g.with the reference mirror oscillating along the optical axis. The rangeof the oscillation is set to be either as great as one wavelength of thelight employed or over the entire coherence length, but no greater thanthe coherence length.

A small range of the modulation, i.e. over one wavelength of the lightemployed, can for example be realized in that the mirror is excited bymeans of a piezoelectric element to execute oscillations. If a largerange of modulation is desired, this can advantageously be achieved bymultiple reflection in a mirror system with a stationary and anoscillating mirror with a small range of modulation.

A time modulation of the optical path length of the test beam orreference beam is also possible by an alteration of the refractive indexof a medium (80 (FIG. 1B) in the beam path, e.g. by electroopticalmodulators, or by a periodic frequency variation of the light in a partof the beam path this brings about in each instance a modulation of theoptical path length difference between reference beam and test beam asfar as the boundary area to be investigated. The modulator 800 (FIG. 1C)could also be implemented using a Bragg cell.

The modulation frequency is selected according to the specifications ofthe modulators employed and of the electronic evaluation system, to befor example between 500 Hz and 500 kHz. If, by way of example, themodulator employed is a mirror excited by a piezoelectric element toexecute oscillations, the greatest possible oscillation frequency of thepiezoelectric element represents the upper limit of the modulationfrequency.

The position-sensitive detection system employed is preferably a lateraldiode (e.g. from the company SiTek Electro Optics or the companyHamamatsu). Its specifications permit a lateral resolution and thusresolution of the deviation of the center of gravity of theinterferogram in relation to the reference axis in the order ofmagnitude of 10 nm. However, CCD cameras are also suitable asposition-sensitive detection systems. However, these have a lateralresolution of only approximately 15 μm, which can be improved, even withmathematical methods, at best to only 150 nm.

Since the process is suitable for being able to distinguish theinterferograms of boundary areas with radii of curvature lying closelyone behind the other in a reliable manner, the inclinations of theboundary areas may be determined even in complex optical systems insuccession. From the deviation of the inclinations of the individualareas from the predetermined optical axis of a total system, it ispossible to obtain a statement concerning the centering condition of thesystem. By controlled decentering of individual areas, it is then alsopossible to correct the centering condition of the total system.

The interferometer construction employed is similar to the constructionaccording to MICHELSON.

Referring to FIG. 1A, a light beam of selected short coherence length,which light beam is emitted by a light source 1, is directed parallel byan auxiliary optical system 2 and is conducted to a beam splitter 3. Thelight beam is split by the beam splitter 3 into two partial beams; inthis case, in the text which follows, the partially reflected componentof the light beam is designated as the test beam and the transmittercomponent is designated as the reference beam.

The reference beam is deflected by means of a first prism system 13 to areference mirror 4, which is disposed perpendicular to the optical axis11 of the reference beam and thereby throws back the reference beam intoitself. The reference beam reflected at the reference mirror 4 passesvia the first prism system 13 and a second prism system 14 onto aposition-sensitive photodetection system 10. As described above, theportion sensitive photodetection system 10 may preferably compromise alateral diode.

Behind the beam splitter 3, by means of the optical collimating system 5the test beam is imaged onto the center of radii 15 of a boundary area 6of an optical system 7, which boundary area is spherical and is in thiscase selected for investigation. The wavefront which is reflected at theboundary area 6 to be investigated and which was designated hereinbeforeas the signal beam is imaged successively via the optical collimatingsystem 5, the beam splitter 3,the auxiliary optical system 2 and prism14, onto the position-sensitive photodetection system 10. The opticalcollimating system 5 advantageously employed in this case is a system ofvariable focal length, in order to collimate onto the various boundaryareas of the system for investigation. Otherwise, the optical system 7to be investigated or the optical collimating system 5 would have to bedisplaced along the optical axis of the test beam.

It proves to be particularly advantageous if, as in the presentembodiment, the reference beam fully covers the reception area of theposition-sensitive photodetection system, since then in all instances aninterference of the signal beam and reference beam is possible on thereception area of the position-sensitive photodetection system 10.

In the example shown, the boundary area is ideally centered with respectto the optical axis of the test beam. The signal beam is reflected intoitself, and the interferogram is created in the center of the detectionsystem 10. As soon as the boundary area is inclined in relation to theoptical axis of the test beam, the reflected signal beam also extends inan inclined manner in relation to this axis, and the interferogram has adeviation in relation to the center of the detection system 10.

In order to balance the optical path length of the reference beam withthat of the test beam as far as the boundary area 6 to be investigated,the reference mirror 4 has an adjustment device 9, which is known per seand thus is here shown only diagrammatically.

In the present embodiment, the optical path length of the reference beamis modulated. As a result of this, the optical path length differencebetween the test beam and the reference beam is modulated. To this end,advantageously the reference mirror 4 itself is integrated into theoptical modulator 8, in that after balancing of the optical path lengthsof the reference beam and test beam to the boundary area to beinvestigated it is set into oscillations along the optical axis 11. Theoscillation-exciting system can be, for example, a piezodrive 16. Thisdrive is suitable in particular for a small range of modulation in theorder of a single wavelength above the light employed. The unitcomprising reference mirror 4 and piezodrive 16 then forms the modulator8.

The oscillation of the reference mirror then takes place about a zeroposition, which is defined by the effected balancing of the optical pathlength of the reference beam and test beam up to the boundary area to beinvestigated, whereby a modulation of the optical path length differenceof these two beams is undertaken.

In a particularly suitable manner, the light source 1 may be a diodelaser with a short coherence length. By altering the injection current,the coherence length can be controlled in a simple manner. In thismanner, an adaptation of the coherence length at differing spacings ofthe boundary area to be investigated in each instance to the boundaryareas reflecting the disturbance beams is possible.

To the position-sensitive photodetection system 10 there is connected anelectronic evaluation system 12 which processes the output signals inthe already described manner particularly advantageously using thelockin technique and indicates as a result the inclination of theboundary area 6 to be investigated in relation to a reference axis.

The selection of the reference axis is without restriction. In practice,in the investigation of an optical system the deviation of thereflection of a specified boundary area is defined as zero deviation.The direction of propagation of the wavefront generating this reflectionthen represents the reference axis.

An alternative method for the selection of the reference axis consistsin setting the optical system to be tested into rotation about itsmechanical axis. Collimation takes place onto a boundary area andobservation takes place of the rotation of the associated interferogramon the reception area of the position-sensitive photodetection system.The center of the rotation represents the zero deviation and thus thereference axis.

We claim:
 1. A method of measuring inclinations of boundary areas in anoptical system, in which a light beam is collimated onto a boundary areawhich is to be investigated in each instance and, from a deviation of awavefront reflected by the boundary area to be investigated in relationto a reference axis on a position-sensitive photodetection system, theinclination of the boundary area to be investigated in each instance inrelation to the reference axis is determined, the method comprising thesteps of:(a) providing an optical system to be tested, the opticalsystem having a boundary area; (b) splitting a light beam into a testbeam and a reference beam; (c) guiding the reference beam via areference beam path onto a reference mirror; (d) matching an opticalpath length of the reference beam to an optical path length of the testbeam to the boundary area to be investigated; (e) selecting a coherencelength of the light beam to be shorter than a shortest optical pathlength between the boundary area to be investigated in each instance anda closest disturbance-reflection-generating boundary area of the opticalsystem; (f) time-modulating an optical path length difference betweenthe reference beam and the test beam to the boundary area to beinvestigated with a range of modulation not greater than a magnitude ofthe coherence length; (g) imaging a wavefront reflected by the boundaryarea to be investigated in each instance onto the position-sensitivephotodetection system where it is heterodyned with a wavefront reflectedby the reference mirror, producing on the position-sensitivephotodetection system a temporally intensity-modulated interferogram;and (h) electronically evaluating position-dependent signals of theposition-sensitive photodetection system to separate a modulated signalcomponent of the temporally intensity-modulated interferogram fromunmodulated signal components generated by disturbance reflections andthe reference beam and thereafter determining a deviation of a center ofgravity of the interferogram in relation to the reference axis tothereby determine the inclination of said boundary area.
 2. A methodaccording to claim 1, wherein a range of modulation of the optical pathlength difference between the reference beam and the test beam as far asthe boundary area to be investigated equals a single wavelength of thelight.
 3. A method according to claim 1, wherein a range of modulationof the optical path length difference between the reference beam and thetest beam as far as the boundary area to be investigated is equal to thecoherence length.
 4. A method according to claim 1, wherein thetime-modulating step comprises the step of periodically varying a pathlength of the test beam or the reference beam.
 5. A method according toclaim 1, wherein the time-modulating step comprises the step of alteringa refractive index of a medium in a beam path of one of the test beamand the reference beam.
 6. A method according to claim 1, wherein thetime-modulating step comprises the step of periodically varying afrequency of light in a part of a beam path of one of the reference beamand the test beam.
 7. A method according to claim 1, wherein thewavefront reflected by the reference mirror covers an entire receptionarea of the position-sensitive photodetection system.
 8. A methodaccording to claim 1, wherein the intensity-modulated components areseparated from the unmodulated components of the position-dependentsignals of the position-sensitive photodetection system using a lock-intechnique.
 9. A method according to claim 1, wherein all boundary areasof the optical system are collimated onto in succession and a centeringcondition of the optical system is determined from an evaluation of theassociated signals of the position-sensitive photodetection system. 10.A device for measuring inclinations of a boundary area in an opticalsystem, comprising:a light source; a position-sensitive photodetectionsystem; an optical imaging system which collimates a light beam, emittedby the light source, onto the boundary area of the optical system andimages a wavefront, reflected from the boundary area onto theposition-sensitive photodetection system; an electronic evaluationsystem connected to the position-sensitive photodetection system, theelectronic evaluation system determining a deviation of a wavefrontreflected by the boundary area in relation to a reference axis and thusof the inclination of the boundary area in relation to the referenceaxis; an auxiliary optical system which generates a parallel light beamfrom radiation of the light source and images the wavefront reflected bythe boundary area onto the position-sensitive photodetection system; abeam splitter for splitting the parallel light beam into a test beam anda reference beam; a reference mirror perpendicular to an optical axis ofthe reference beam; an adjustment device for adjusting an optical pathlength of the reference beam to equal an optical path length of the testbeam to the boundary area; a coherence length of the light source beingshorter than a shortest optical path length between the boundary areaand a closest disturbance-reflection-generating boundary area of theoptical system; a modulator, the modulator time-modulating the opticalpath length of the test beam or the reference beam with a range ofmodulation no greater than a magnitude of the coherence light; and a twoprism system which returns a wavefront, reflected by the referencemirror, to the position-sensitive photodetection system such that anintensity-modulated interferogram of interference between the test beamand the reference beam is produced, wherein the electronic evaluationsystem processes position-dependent signals output by theposition-sensitive photodetection system to separate a modulated signalcomponent of the intensity-modulated interferogram from unmodulatedsignal components generated by disturbance reflections and the referencebeam, and thereafter determines the deviation of a center of gravity ofthe interferogram in relation to the reference axis.
 11. A deviceaccording to claim 10, wherein the position-sensitive detection systemcomprises a lateral diode.
 12. A device according to claim 10, whereinthe modulator comprises a reference mirror coupled with a piezodrive.13. A device according to claim 10, wherein the modulator comprises aBragg cell.
 14. A method of measuring an inclination of a boundary areaof an optical system, the optical system further including a disturbanceboundary area, the method comprising the steps of:providing an opticalsystem to be tested, the optical system having a boundary area and adisturbance boundary area; generating a beam of electromagneticradiation; splitting the beam into a test beam and a reference beam;collimating the test beam onto the boundary area of the optical system;imaging a wavefront reflected from the optical system onto aposition-sensitive photodetection unit; directing a wavefront of thereference beam to the position-sensitive photodetection unit where thewavefront of the reference beam interferes with the wavefront reflectedfrom the optical system; time modulating an optical path lengthdifference between a path length of the test beam and a path length ofthe reference beam, such that the wavefront of the reference beaminterferes with a component of the wavefront reflected from the opticalsystem corresponding to a reflection from the boundary area of theoptical system while the wavefront of the reference beam does notinterfere with a component of the wavefront reflected from the opticalsystem corresponding to a reflection from the disturbance boundary areaof the optical system, to produce an intensity modulated interferencepattern associated with the component of the wavefront reflected fromthe optical system corresponding to the reflection from the boundaryarea of the optical system; electronically evaluating output signals ofthe position-sensitive photodetection unit to extract the intensitymodulated interference pattern; and determining the inclination of theboundary area from a position of the intensity modulated interferencepattern.